Polyethylene is the most widely used commercial polymer. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).
A normal set-up of a plant for preparing low density polyethylene comprises, a polymerization reactor which can be an autoclave or a tubular reactor or a combination of such reactors and additional equipment. For pressurizing the reaction components, usually a set of two compressors, a primary and a secondary compressor, is used. At the end of the polymerization sequence, a high-pressure polymerization unit that may include apparatuses like extruders and granulators for pelletizing the resulting polymer can be used. Furthermore, such a polymerization unit generally also comprises means for feeding monomers and comonomers, free-radical initiators, modifiers or other substances at one or more positions to the polymerization reaction.
A characteristic of the radically initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is generally not complete. For each pass of the reactor, only about 10% to 50% of the dosed monomers are converted in the case of polymerization in a tubular reactor, and from 8% to 30% of the dosed monomers are converted in the case of polymerization in an autoclave reactor. Accordingly, it is common practice to separate the discharged reaction mixture into polymeric and gaseous components and recycle the monomers. To avoid unnecessary decompression and compression steps, the separation into polymeric and gaseous components is usually carried out in two stages. The reaction mixture leaving the reactor is transferred to a first separating vessel, frequently called a high-pressure product separator, in which the separation in polymeric and gaseous components is carried out at a pressure that allows for recycling of the ethylene and comonomers separated from the reaction mixture to the reaction mixture at a position between the primary compressor and the secondary compressor. At the conditions for operating the first separation vessel, the polymeric components within the separating vessel are in a liquid state. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, frequently called a low-pressure product separator, in which a further separation into polymeric and gaseous components takes place at lower pressure. The ethylene and additional comonomers separated from the mixture in the second separation vessel are fed to the primary compressor, where they are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the joined streams are further pressurized to the pressure of the high-pressure gas recycle stream. The level of the liquid phases in the first and the second separating vessels are generally measured by radiometric level measurement and are controlled automatically by product discharge valves.
Separating polymeric and gaseous components of a reaction mixture obtained by high-pressure polymerization of ethylenically unsaturated monomers in the presence of free-radical polymerization initiators by lowering pressure and temperature has already been described in GB 580182. Apparatuses of different design and geometry have been disclosed as low pressure separating vessels operating in a pressure range of from 0.12 MPa to 0.6 MPa. DE 1219378 describes low pressure separators, in which polyethylene is homogenized by intensive stirring. DD 1219378 refers to a two-stage product separation process, in which an low pressure separator is employed into which the polymer melt is introduced at a point which is at 60% to 100% of the total height of the low pressure separator, through a pipe which is directed downwards at an angle of 50° to 70° to the horizontal close to the wall of the vessel. Preferably the melt inlet is at 60% to 80% of the total height of the low pressure separator. WO 2011/078856 A1 discloses a low pressure separation vessel which includes a cylindrical section that ends in a frustroconical section at its bottom and which has a product inlet mounted on its side for receiving the polymer product from the high pressure separator.
Processes for separating polymeric and gaseous components of a composition obtained by high-pressure polymerization of ethylenically unsaturated monomers allow for the recycling of ethylene, comonomers and other low-molecular weight components from the reaction mixture to the suction side of the secondary compressor. However, there are still a considerable amount of polymeric components carried over by the gas stream leaving the second separating vessel. These components have to be separated from the gas in subsequent separation steps in the ethylene recycle line. Furthermore, entrained polymeric components can adhere to the surface of the separation vessel or to the surface of transfer conduits and cause fouling.
Accordingly there is a need in the art to overcome the disadvantages of the prior art and to provide separation processes which show a very low amount of polymer carry over with the gas stream leaving the second separation vessel. Furthermore, there is a need in the art for processes that allow longer periods of production without the necessity of cleaning the separation vessel and a fast grade change between the different types of produced, low density polyethylenes with a reduced amount of out-of-specification polymer. In addition, since the separating vessels in high-pressure processes for preparing olefin polymers in the presence of free-radical polymerization initiators are large, pressure-resistant apparatuses, there is a constant desire for economic reasons to be able to construct such separation vessels as small as possible without losing a good separating efficiency.